The association of diabetes mellitus with microvascular disease and accelerated macrovascular disease is a serious problem for the juvenile-onset diabetic patient. The contribution of poor diabetic control to microvascular and macrovascular disease is not clear at the present time. A prospective evaluation of the role of diabetic blood glucose control in the development of these complications requires adequate and precise assessment of blood glucose levels over an extended time period. It is proposed that serial measurements of glycosylated hemoglobins will provide an accurate integrated index of blood glucose levels in the diabetic patient over an extended period of time. This hypothesis will be tested with a followup of 100 juvenile diabetic patients over a five year period. The association of such indices of blood glucose control will be correlated with the development and progression of microvascular disease. Quadriceps capillary basement membrane thickness obtained on muscle biopsies at the beginning of the study, and two and four years later, will be used to assess the progression of microvascular disease during the study period. Both indices of diabetic blood glucose control, and the progression of capillary basement membrane thickness will be correlated with the traditional risk factors for the development of macrovascular disease in this population. In addition, the profiles of risk factors for macrovascular disease in the diabetic patients will be compared to similar profiles in a non-diabetic sibling population to determine whether the usual risk factors are mediators of the accelerated atherogenesis seen in diabetes.